Temperature control of ore in multiple hearth furnace

ABSTRACT

The temperature of ore on the hearth of a multiple hearth furnace is continuously monitored by means of a thermometer positioned in the undisturbed dead bed of the ore beneath the live bed which is being continuously raked. The temperature readings are used to control the roasting temperature in the live bed within a preselected range.

D United States Patent [151 3,658,309 Lavender [451 Apr. 25, 1972 [54]TEMPERATURE CONTROL OF ORE IN [56] References Cited MULTIPLE HEARTHFURNACE UNITED STATES PATENTS Inventor: William James Lavender, E m n n,2,333,111 11/1943 Lykken ..266/20 berta, Canada 2,302,841 1 l/ 1942Connolly. I Assisnee: Sherri" Gordon Mines Limited Tommo, 1,808,5076/1931 Poole ..73/343 Oman), Canada Primary ExaminerLe0nidaS Vlachos 2Filed; Aug. 3 970 At!orney--Frank l. Piper, Arnie I. Fors and James T.Wilbur 211 Appl. No; 60,668 57 ABSTRACT The temperature of ore on thehearth of a multiple hearth fur- [52] US. Cl ..266/20, 73/343, 263/26nace istcontinuously monitored by means of a thermometer [51] F27b 21/00positioned in the undisturbed dead bed of the ore beneath the [58] Fieldof Search ..266/20; 73/343; 263/26 live b which is being continuouslyraked- The temperature readings are used to control the roastingtemperature in the live bed within a preselected range.

6 Claims, 2 Drawing Figures PATENTEDAPR 25 I972 3, 658. 309

In venlor WILJJAM J. LAVENDER TEMPERATURE CONTROL OF ORE IN MULTIPLEHEARTH FURNACE 1 This invention relates to a method and apparatus formeasuring ore temperature and more particularly to a method andapparatus for measuring the temperature of an ore disposed upon a hearthin a multiple hearth furnace.

Ores which are treated by hydrometallurgical or pyrometallurgicalprocesses for the extraction of valuable metals are frequently subjectedto a preliminary roasting operation in order to increase the amenabilityof the ores to the subsequent processes. The ores may be roasted toreduce certain or all metal values, to oxidize metal values or to adjustthe moisture content. Roasting is commonly carried out in a multiplehearth furnace such as a Herreshoff furnace which consists of a seriesof superimposed circular hearths. A central rotating shaft turns rabblearms that extend out across the hearths. The rabbles are set at anglesthat work the ore outward or inward on alternate hearths, turning theore over to bring new surfaces in contact with gas moving up through thefurnace. The ore is fed into the topof the furnace and falls to thefirst hearth where it is moved inward and falls through a drop hole atthe center of the hearth to the hearth below where it is moved outwardto a drop hole near the perimeter of the hearth, inward on the nexthearth and so on down. As the ore passes downward, it is heated byrising hot gases.

The ore on the hearths is heated by the rising hot gases evolved fromthe combustion of fuel used to fire the furnace. The temperature of theore is controlled by regulating the quantity of fuel used or byregulating the flow of air into the furnace at the lower hearth levels.

It is essential that the temperature of the ore on the hearths bemaintained within a preselected range to ensure satisfactory roasting.The temperature range will of course be dependent on the nature of theore, its grain size, the metallurgical process which follows theroasting operation and other factors. For example, if 1 mm pyrrhotitegrains are subjected to an oxidizing roast, the temperature of thegrains must be sufficient to ignite the grains, generally 430 C. Thetemperature must however be below the fusing point of the pyrrhotite.Lateritic ores which are subjected to a preliminary reduction roastbefore being subjected to a conventional leaching operation in anaqueous ammoniacal ammonium carbonate solution also require closetemperature control in the furnace. The temperature must be such thatnickel and cobalt oxides in the ore reduce to a metallic state leachablein the carbonate solution with a minimum accompanying reduction of ironoxide in the ore to metallic iron and ferrous iron. If the grain size ofthe lateritic ore is substantially 100 percent minus 100 mesh standardTyler screen, the temperature of the ore within the furnace should bemaintained in the range of 500 to 750 C. and the temperature of the oreon each hearth within narrow predetermined limits, e.g., 575 1 C. tobring about the desired reduction.

In order to maintain the temperature of the ore on the hearths withinthe desired range, means must be provided for determining the oretemperature. A number of methods for determining the ore temperature ona periodic basis are currently in use. One method involves measuring thetemperature of the gas within the furnace. Such a measurement may bereadily made but in most cases the gas temperature is significantlyhigher than the temperature of the ore on the hearths and the gastemperature can rarely be directly related to the temperature of theore.

Another method involves the periodic taking of a sample of the ore andmeasuring the temperature of the sample. A cup is mounted in a drop holethrough which the ore passes as the ore falls from one hearth to thehearth below. A sheathed thermocouple is mounted in the cup and the cupis connected to an elongated handle which projects through the wall ofthe furnace. The other end of the handle is outside the furnace and maybe manually rotated in order to turn the cup from an upturned to adownturned position. Samples of ore falling through the drop hole inwhich the cup is positioned will be captured in the cup when the cup isin an upturned position. The temperature of the ore sample within thecup is measured by the thermocouple.

There are a number of problems associated with this method of measuringthe temperature of the ore. When the cup is in a downturned position,its temperature will rise until it reaches the temperature of the gas towhich it is exposed. As a sheathed thermocouple requires a minute orlonger to arrive at a stabilized temperature, the stabilized temperaturewill be the same as or very close to the temperature of the gas. Whenthe cup is turned over and a sample of ore falls into the cup, thetemperature of the thermocouple will begin to drop because the ore iscooler than the gas. However, by the time the temperature readingapproaches the temperature of the ore, the temperature of the ore samplewill be about the same as the temperature of the gas because the sampleis heat conductive and is surrounded by the hot gas. Thus thetemperature at the thermocouple begins to rise again and will stabilizeat the gas temperature. This method is therefore unsatisfactory foraccurately measuring the temperature of the ore within the furnace.

It is accordingly an object of the present invention to pro vide amethod and apparatus for determining accurately the temperature of oreon the hearths of a multiple hearth furnace.

Another object is to provide an'economical and simple method andapparatus which permits the continuous monitoring of the temperature ofore on the hearths of a multiple hearth furnace.

Broadly, the present invention may be considered to involve animprovement in the method of heating an ore disposed on a hearth of amulti-hearth furnace wherein the temperature is controlled within apreselected range to effect roasting of the ore, the ore being disposedin two beds having a common interface and comprising a lower dead bedand an upper live bed, the dead bed remaining relatively undisturbed andthe upper bed being continuously raked. The improvement in the methodinvolves placing a thermometer in the dead bed and utilizing thetemperature determination to control the roasting temperature of the orein the live bed within the preselected temperature range.

According to an alternative aspect, the invention is for use in afurnace having a hearth disposed within the furnace for supporting anore; a rabble arm disposed immediately above the hearth having aplurality of downwardly extending teeth for raking the ore. Theapparatus of the invention comprises a thermometer having a temperaturedetecting element and connected to the hearth such that the detectingelement is disposed above the hearth but beneath the teeth of the rabblearm.

The ore on the hearths of a multiple hearth furnace is divided into twobeds, an upper live bed and a lower dead bed. The live bed is raked bymeans of rotating rabble teeth and is constantly in motion. The lowerdead bed lies beneath the lowermost edges of the rabble teeth andremains relatively undisturbed. Ore from the live beds forms the bulk ofthe furnace discharge; insignificant amounts of charge from the deadbeds are discharged from the furnace. Accordingly, the roastingtemperature of the charge in the live beds must be controlled. It ishowever impractical to insert a temperature measuring device into thelive bed because the device would be struck by a rabble tooth andrendered inoperative.

It has hitherto been believed that the temperatures of the chargeforming the live and dead beds were very different. The live bed wasbeing constantly turned over by the rabble teeth in order to expose allparticles to the hot furnace gases whereas the particles of the dead bedremained stationary and out of contact with the gases. It was thereforebelieved that the temperature of the live bed was much different thanthe temperature of the dead bed. It has been discovered however that thetemperature of the live and dead beds of most furnace ores are much thesame. Metalliferous ores which are moderately to highly heat conductiveform into live and dead beds of much the same temperature in a multiplehearth furnace. Nickeliferous sulphide and oxide ores includinggarnieritic and sepentinic laterites are examples of such metalliferousores. Non-metallic ores such as coal and calcium carbonate are alsomoderately to highly heat conductive and temperatures in the live anddead beds of such ores in a multiple hearth furnace are very similar.

Accordingly, the temperature of the live bed of an ore within a multiplehearth furnace can in most cases be obtained by measuring thetemperature of the dead bed. Since the ore particles in the dead bedremain stationary, their temperature can be readily measured by means ofconventional temperature measuring devices such as a thermocouple, dialthermometer, and resistance bulb.

A fuller understanding of the invention may be had by referring to thefollowing description of a preferred embodiment of the present inventiontaken in conjunction with the accompanying drawing, in which:

FIG. 1 is a view of a Herreshoff furnace and thermometer according tothe invention partly in section and FIG. 2 shows the thermometer in thefurnace.

Like reference characters refer to like parts throughout the descriptionof the drawing.

The furnace illustrated in FIG. 1 is a so-called multi-hearth furnacewhich consists ofa cylindrical metal shell that supports arches offirebrick 12 decked one above the other. A plurality of circular hearths14a, b, c; and d are disposed in decks within the furnace. A verticalcentral rotating shaft 16 running through the center of the brick archescarries rabble arms 18 having a plurality of downwardly extending teeth20. The rabble arms are disposed above the hearths and the teethattached thereto serve to take the furnace charge across the hearth uponrotation of shaft 16.

A hopper 22 is secured to the furnace at the top. Charge to the furnaceis fed to the hopper and passes downwardly through drop hole 24 to theperiphery of hearth 14d. The charge is raked inwardly along the hearthto drop hole 26 where it falls downwardly to hearth 14c. The ore israked outwardly along the hearth and falls through drop hole 28 and soon down.

Shaft 16 is carried on a heavy bearing 30 and is rotated by a bevel gear32 at the bottom, further reduction being obtained by a gear reductionunit 34. The speed of rotation of shaft 16 is determined largely by therequired retention time in the furnace or the depth of bed ofore on thehearths.

In order to maintain a uniform bed on each hearth 14, it is necessarythat the ore be closely sized, 1%. inches being about the maximumsuitable size.

Fuel such as oil is fed to the furnace through conduit 36 and air is fedthrough conduit 38. As the ore travels downward from hearth to hearth,it is thoroughly stirred and exposed to the hot gases evolved duringcombustion of the fuel. The roasted calcine is discharged through exitport 41 in the bottom of the furnace.

Thermometer 42 is secured to hearth 14a and like thermometers aresecured to the remaining hearths.

With reference to FIG. 2, the illustrated thermometer is a thermocouplecomposed of two legs 44 and 46 of dissimilar metals. The legs aremaintained in a fixed spaced relation with one another within tubing 48by means of ceramic rings 50. The free ends of the legs project from theend of tubing 48 and are joined at temperature sensing junction 52.Tubing 48 projects through an aperture 54 formed in wall 56 of thefurnace. The tubing is expanded and ferruled to the aperture wall at 57in order to produce a seal between the tubing and the furnace wall.

Preferably tubing 48 is contoured to the shape of the upper surface ofhearth 14a so that the tubing is in contact therewith. A U-bolt 58,anchored in hearth 14a, serves to prevent the tubing from swinging toeither side or upward upon expansion and contraction of the tubing withchanges in temperature.

Rabble teeth are arranged on arm 18 so that substantially every point onthe surface of hearth 14a is passed over by a tooth. To prevent theteeth from damaging the upper surface of the hearth as the rabble armrotates, the lower edges of the teeth are spaced apart from the hearthupper surface. Changes in temperature within the furnace will producevertical expansion and contraction of shaft 16 and a correspondingmovement of teeth 20 away from and toward hearth 140, thus the teeth andhearth must be arranged such that the teeth, when at their closest pointto the hearth, do not make contact with the hearth.

The ore upon hearth 14a is divided into two layers, an upper live bed 60which is raked by teeth 20 and a lower dead bed 62 which is notdisturbed by the teeth. The thickness of the dead bed will be thedistance between the lower edges of the teeth and the upper surface ofthe hearth when the teeth are closest to the hearth. Usually thethickness of the dead bed 62 is about 2 to 5 inches.

In many cases, the temperature of the ore in the live and dead beds isuniform and therefore junction 52 may be located anywhere in the deadbed. In some cases however, the temperature is not constant in the twobeds. The gases within the furnace greatly influence the temperature ofthe upper surface of the live bed but have much less effect on thetemperature of the head bed and the temperature will vary from the uppersurface of the live bed to the lower surface of the dead bed.

The most suitable location of junction 52 is directly beneath theinterface between beds 60 and 62 where the junction is not damaged bythe rabble teeth. The temperature measured at the junction will be closeto if not the same as the temperature of the lower zone of the live bed.

The thermometer must be capable of withstanding temperatures of the deadbed. Where the dead bed is corrosive to the thermometer, protection inthe form of sheathing is required. The thermometer may be a thermocouplecomprising a platinum wire and a wire of some other refractory metalsuch as iridium or an alloy of platinum and iridium, rhodium orchromium. Preferably the thermocouple is composed of platinum and analloy composed of 87 percent platinum and 13 percent rhodium. Thesemetals are capable of standing up to highly corrosive reducingatmospheres for long periods of time without appreciable deterioration.The thermocouple may or may not be shielded. When the temperature of thecharge is subject to rapid changes, an exposed thermocouple is preferredsince it is capable of reaching equilibrium at a new temperature morequickly than a shielded thermocouple.

The tubing may be constructed of stainless steel or other material whichis not corroded by the furnace charge.

The improved roasting method of the present invention is especiallyapplicable to lateritic ores. In order to reduce the nickel and cobaltoxides in the lateritic ore to metallic state with a minimumaccompanying reduction of iron oxide to metallic iron and ferrous iron,the ore is roasted in the presence of reducing gases such as hydrogen orcarbon monoxide or mixtures thereof. The temperature of the ore in thelive beds within the furnace is continuously monitored by means ofthermometers positioned in the dead beds. The temperature of the livebed on the hearth is maintained within narrow predetermined limits byregulating the quantity of fuel fed to the furnace or by regulating theflow of air into the furnace.

I claim:

1. In the method of heating an ore disposed on a hearth ofa multi-hearthfurnace wherein the temperature is controlled within a preselected rangeto effect roasting of said ore, said ore being disposed in two bedshaving a common interface and comprising a lower dead bed and an upperlive bed, said dead bed remaining relatively undisturbed and said upperbed being continuously raked, the improved procedure for continuouslydetermining the temperature of the ore in said live bed characterized inthat a thermometer is placed in the dead bed and said temperaturedetermination is utilized to control the roasting temperature of the orein said live bed within said preselected temperature range.

2. The method as claimed in claim 1 wherein said thermometer is placedin the dead bed adjacent said interface.

3. The method as claimed in claims 1 or 2 wherein said ore is alateritic ore.

4. In a furnace having a hearth disposed within said furnace forsupporting an ore, a rabble arm disposed immediately above said hearthhaving a plurality of downwardly extending teeth for raking said ore theimprovement comprising: a thermometer having a temperature detectingelement and being connected to said hearth such that said detectingelement is disposed above said hearth but beneath the teeth of saidrabble arm.

5. In a furnace having a hearth disposed within said furnace forsupporting an ore, a rabble arm disposed immediately above said hearthhaving a plurality of downwardly extending teeth for raking said ore,said ore when supported by said hearth being disposed in two beds, alower dead bed and an upper live bed, said beds having a commoninterface and being at substantially the same temperature, said dead bedremaining relatively undisturbed and said live bed being raked by meansof said teeth, the improvement comprising a thermometer having atemperature sensing element, said sensing element being maintainedwithin said dead bed adjacent said interface.

6. The combination as claimed in claim 5 wherein said thermometer is athermocouple composed of two dissimilar metals, one said metal beingplatinum and the other said metal being an alloy composed of about 87percent platinum and the balance rhodium.

1. In the method of heating an ore disposed on a hearth of amulti-hearth furnace wherein the temperature is controlled within apreselected range to effect roasting of said ore, said ore beingdisposed in two beds having a common interface and comprising a lowerdead bed and an upper live bed, said dead bed remaining relativelyundisturbed and said upper bed being continuously raked, the improvedprocedure for continuously determining the temperature of the ore insaid live bed characterized in that a thermometer is placed in the deadbed and said temperature determination is utilized to control theroasting temperature of the ore in said live bed within said preselectedtemperature range.
 2. The method as claimed in claim 1 wherein saidthermometer is placed in the dead bed adjacent said interface.
 3. Themethod as claimed in claims 1 or 2 wherein said ore is a lateritic ore.4. In a furnace having a hearth disposed within said furnace forsupporting an ore, a rabble arm disposed immediately above said hearthhaving a plurality of downwardly extending teeth for raking said ore theimprovement comprising: a thermometer having a temperature detectingelement and being connected to said hearth such that said detectingelement is disposed above said hearth but beneath the teeth of saidrabble arm.
 5. In a furnace having a hearth disposed within said furnacefor supporting an ore, a rabble arm disposed immediately above saidhearth having a plurality of downwardly extending teeth for raking saidore, said ore when supported by said hearth being disposed in two beds,a lower dead bed and an upper live bed, said beds having a commoninterface and being at substantially the same temperature, said dead bedremaining relatively undisturbed and said live bed being raked by meansof said teeth, the improvement comprising a thermometer having atemperature sensing element, said sensing element being maintainedwithin said dead bed adjacent said interface.
 6. The combination asclaimed in claim 5 wherein said thermometer is a thermocouple composedof two dissimilar metals, one said metal being platinum and the othersaid metal being an alloy composed of about 87 percent platinum and thebalance rhodium.